Environmentally benign sorbents for removing mercury from flue gas

ABSTRACT

A new class of carbon-based sorbents for vapor-phase mercury removal is disclosed in this invention. The optimum structure of the sorbent particles, and a method to produce the sorbent, are described. The sorbent is based on carbon particles with a metal-oxide coating on the surface. The thin metal-oxide layer acts as a barrier for the adsorption of Air Entrainment Admixture (AEA), the component used to stabilize bubbles in cement), thereby enhancing its concrete friendliness. The metal-oxide is coated on the surface of carbon, using a solution-based method. The metal-oxide coated carbon was further modified with sulfur molecules, to increase its mercury removal capacity.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. provisional application No.61/215,887 filed May 11, 2009.

STATEMENT OF GOVERNMENT SUPPORT OF INVENTION

The work leading to the present application was done as part of DOEGrant Number: DE-FG02-07ER84714.

BACKGROUND OF THE INVENTION

This invention relates to the use of certain nanoscale particles as asorbent to remove mercury from flue gas. Under the EPA's Clean AirMercury Rule, coal fired power plants are required to drastically reducethe amount of mercury (Hg) emissions within the next several years. Oneof the technologies under consideration for removal of Hg is the use ofchemically treated (brominated) activated carbon. It has been noted thatutility companies may need to take into account the impact of a recentcourt decision, which specifies that a power plant cannot implement amercury control solution that could potentially increase the amount of asecondary pollutant, unless additional controls for that pollutant areinstalled. This could be an issue in the case of brominated activatedcarbon, as bromine emissions can have adverse environmental effects.Compared to chlorine, bromine is considered to be more potent indepleting the atmospheric ozone layer. There are additional corrosionissues related to the presence of bromine in the system. Further, amajority of these activated carbons are not concrete-friendly, i.e. thefly ash containing the activated carbon particles cannot be used inconcrete. This leads to loss of revenue to the power plants on twocounts: (i) loss of revenue due to lack of usability of fly ash; and(ii) cost of disposing unusable fly ash in landfills. Additionally, theuse of fly ash has an important consequence for the environment: if allof the fly ash produced can be used as replacement for cement, it canreduce CO₂ emissions equivalent to that generated by 25% of the world'sautomotives.

Coal fired power plants constitute ˜52% of the total electricityproduced in the United States. As the demand for electricity increases,utility companies are increasing the generating capacity as well.Additionally, many of the current nuclear plants will be “retired” inthe first quarter of the 21^(st) century. Due to poor public support fornuclear energy, these nuclear plants are likely to be replaced by coalfired plants. At the current consumption rate, it is estimated that theworld has ˜1500 years of coal reserves. This leads to the recent steadyincrease in the amount of coal consumed in the world and in the US. Thisimplies that the mercury emission issue associated with coal-fired powerplants needs to be resolved in the long run.

An estimated total of 48 tons of mercury is emitted every year in the USfrom coal-fired power plants, which is ⅓^(rd) of the total mercuryemissions per year in the US. On a worldwide scale, this is a muchlarger issue, since countries such as China and India are usingincreasing amounts of energy derived from fossil fuels. Under the ClearSkies Initiative, the target is to reduce mercury emission by about 45%by 2010, and about 70% by 2018. New technologies will need to bedeveloped to reach these targets. According to DoE, the marketpenetration for mercury emission reduction technologies is an estimated320,000 megawatts. In order to achieve the target reduction by 2018, theadditional annual cost for energy generation will be $2 billion to $6billion per year, if the existing activated carbon (current estimate is$18,000-$131,000 per pound of mercury removal, using activated carbontechnology.

A major issue is the usability of fly ash containing mercury adsorbedactivated carbon (it cannot be used if the mercury content is high),which further increases the cost of using activated carbon technologyfor mercury removal. Fly ash is a valuable by-product from coal-firedpower plants. In making concrete, cement is mixed with water to act asan adhesive to hold strong aggregates. Fly ash is added during theprocess, as it is observed that concrete containing fly ash is easier towork with, and it uses 10% less water. Additionally, fly ash reacts withlime that is given off by cement hydration, creating more bonding agentto hold the concrete together, which makes concrete stronger with time,compared to concrete without fly ash. Further, it reduces the amount ofcement required to make concrete. While a ton of cement costs $80-$100,fly ash costs only $32/ton making it more competitive than cement.Manufacturing one ton of cement requires 6.5 million BTUs of energy, andit is estimated that cement plants produce 7% of the total CO₂ emissionby human sources. If all the fly ash produced can be used to partlyreplace cement in concrete, it can eliminate CO₂ emissions equivalent tothat of 25% of the automotives in the world. Clearly, there areenvironmental and societal benefits that are derived from lower mercuryand CO₂ emissions. Also, the use of fly ash will save landfill space.However, even the presence of less than 1% of activated carbon in flyash can make it useless for mixing with concrete, by changing itsproperties.

Therefore, it is imperative that any sorbent used for removing mercuryfrom flue gas be concrete-friendly. Conventional activated carbon is notconcrete-friendly, and most brominated activated carbons are notconcrete-friendly either. Recently, it has been reported that somebrominated activated carbon may be concrete friendly, but the negativeenvironmental effects of bromine are yet to be studied and not known atthe moment. Additionally, bromine is a highly corrosive gas, and as suchthe impact on the exhaust ducts could be a problem.

Currently, various types of activated carbons are being extensivelystudied for mercury removal from flue gas. DOE/NETL has carried outseveral field tests of activated carbons due to their high removalefficiency. Three prominent brands of activated carbons which have beentested in the field are NORIT Americas (Darco® Hg-LH), Alstom PowerPlant Laboratories (Mer-Clean™), and Sorbent Technologies Corporation(B-PAC™). Results indicated that activated carbon consistently performedwell in mercury removal, on a full-scale test. However, secondarypollution (bromine), corrosion from bromine and concrete friendliness isstill an issue, affecting their overall performance.

Another media which is used to remove mercury from flue gas is based on“clay”, and is manufactured by Amended Silicates. However, when theperformance of this media was compared with various types of activatedcarbon sorbents the amended silicate media did not perform as well asactivated carbon. Others used a fluidized bed of zeolite and activatedcarbon for the removal of organics and metals form gas streams. Zeolitesare aluminosilicate materials that are extensively used as adsorbentsfor gas separation and purification, and they are also used asion-exchange media for water treatment and purification. Zeolites have“open” crystal structures, constructed from tetrahedra (TO₄, where T=Si,Al). It has been observed that the removal efficiency of metals presentin gases by activated carbon is higher than that of zeolite, and thetemperature only slightly influences the removal efficiency. A studytested treated Zeolite and observed 63% mercury removal efficiency.

U.S. Pat. No. 6,610,263 is directed to the use of high surface areaMnO_(x) to remove Hg. It is claimed that it has the capability to remove99% of elemental Hg and 94% of the total mercury content in flue gas.However, the cost is likely to be a concern for using this media inpractical applications.

Biswas et-al [T. M. Owens and P. Biswas, J. Air & Waste Manage. Assoc.,n46, 1996, p 530] have developed a gas-phase sorbent precursor method,where a high surface area agglomerated sorbent oxide particle isproduced in situ in the combustor. These sorbents are stable at elevatedtemperatures and provide a surface of metallic vapors (for condensation)and reaction. They used titanium isopropoxide as precursor, whichdecomposed at elevated temperature and formed particles of titania. Hgvapors were found to condense on these particles in the presence of UVradiation which helps in the oxidation of mercury vapors and formationof a strong bond between mercury and titania. They [P. Biswas and M.Zachariah, “In situ immobilization of lead species in combustionenvironments by injection of gas phase silica sorbent precursors”, Env.Sci. & Tech., v31, n9, 1997, p 2455] also used silica precursors for theremoval of lead from flue gas, and were able to get 80-90% lead removalefficiency. The removal efficiency was found to be a function of the gastemperature. Additionally, the efficiency was observed to decrease withincrease in temperature.

Another group has shown the feasibility of using a fluidized bed for theremoval of metals, such as lead, from flue gas. They used limestone,bentonite, and alumina as sorbents, and observed that the effectivenessof the fluidized bed depends on sorbent species, sorbent particle size,amount of sorbent used, metal to sorbent ratio, metal concentration inthe waste, air velocity, and temperature. Smaller particles showedbetter efficiency compared to larger particles (particle range 400-700μm). In case of limestone, it increased from 60% to 70% when theparticle size was decreased from 700 to 500 μm, all other conditionsremaining same. The sorbents showed better efficiency at lowertemperatures (˜750° C. vs. ˜900° C.). This is because at highertemperatures, the vapor pressure is high, so more metal escapes asvapor.

Still others have used zeolite materials for the removal of mercury byduct injection. They were able to get between 45 and 92% metal removaldepending upon the amount of sorbent injected and the type of sorbent.In the case of zeolites, there was no effect of temperature on theremoval efficiency.

Gullet et-al [B. Gullet and K. Raghunathan, “Reduction of coal basedmetal emissions by furnace sorbent injection”, Energy & Fuels, v8, 1994,p 1068] demonstrated the feasibility of using oxide minerals such aslimestone, kaolinite, and bauxite as sorbents for toxic metal removal,by injecting them through the burner. They were able to get reduction insubmicron size metal particles of antimony, arsenic, mercury, andselenium by hydrated lime and limestone.

SUMMARY OF THE INVENTION

The present invention is directed to sorbents for removing mercury fromgas and their synthesis. The sorbent has a highly accessible surface,and selectivity towards mercury adsorption. These sorbents are halogen(bromine) free, making them environmentally safe as well asnon-corrosive towards the power plant system. Certain of thenon-activated carbon based sorbents, such as zeolites and oxides areconcrete and environment friendly, however their mercury removalefficiency is significantly lower than activated carbon. The presentinvention overcomes the limitations of currently available carbon andnon-carbon based sorbents by incorporation of a barrier layer on thesurface of the particles.

The sorbent described in the present invention is based on carbonparticles with a metal-oxide coating on the surface. The thinmetal-oxide layer acts as a barrier for the adsorption of AirEntrainment Admixture (AEA, the component used to stabilize bubbles incement), thereby enhancing its concrete friendliness. The metal-oxide iscoated on the surface of carbon, using a solution-based method. Themetal-oxide coated carbon was further modified with sulfur molecules, toincrease their mercury removal capacity.

Two critical aspects that differentiate the newly developed carbon-basedparticles from other sorbent particles include: (i) suitable surfacemodification that leads to high affinity for mercury ions without havingto use toxic elements such as bromine, and (ii) the ability to renderthe resulting fly ash usable in concrete and other applications, due tothe low foam index. The overall mercury removal efficiency is comparableto that of the best performing commercial sorbent, which is a brominatedcompound.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference is made to thefollowing drawings which are to be taken in conjunction with thedetailed description to follow in which:

FIG. 1 depicts the mechanism to explain the effect of surface oxidelayer on foam index: (A) the hydrophobic side of AEA molecule (smallcircle) is attracted towards carbon; (B) the hydrophobic side isrepelled from silica coated carbon, due to the presence of negativecharge on the surface of the carbon.

FIG. 2 is a TEM micrograph of coated carbon black

FIG. 3 is a TEM micrograph of the ash, after carbon burnout

FIG. 4 depicts the mercury removal efficiency of sorbents carbon black,the inventive sorbent C2 and Darco Hg-LH;

FIG. 5 depicts the foam index of C2 and Darco Hg-LH

FIG. 6 depicts the mercury removal efficiency of sorbent, the inventivesorbents AC-1, C1, C3, and Darco Hg-LH.

FIG. 7 depicts the foam index of C1, C3 and Darco Hg-LH

FIG. 8 depicts mercury removal efficiency of the inventive sorbent C5and Darco Hg-LH.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Overview

A. The template material used as a sorbent is carbon black and activatedcarbon, as carbon-based materials are readily available. Other types ofcarbon particles that have a similarly open morphology can also be used.Additionally, other non-carbon materials, such as ceramic oxides,ceramic non-oxides, or clay-based particles can also be used as templatefor further surface modification.

B. The surface of the carbon particles was modified using a two-stepprocess. During the first step, the surface was modified with aluminumhydroxide, to form a tie layer. This leads to the activation of thesorbent surface with aluminum hydroxide functional groups. An aqueoussolution of sodium aluminate was used as the precursor for aluminumhydroxide deposition. Sodium aluminate was transformed to aluminumhydroxide, by treating it with an ion-exchange resin. The resinexchanges sodium ions to hydrogen ions. It should be noted that otherinorganic compounds such as: titanium hydroxide, magnesium hydroxide,iron hydroxide, copper hydroxide can also be used as the tie layer priorto deposition of metal oxide layer.

C. The surface of carbon was further modified with a metal oxide, duringthe second step. In our work, we used silica because it is the leastexpensive among oxides and allows for easy surface modification. Othercommonly known oxides, including aluminum oxide, titanium oxide, ironoxide and tin oxide can be used instead. Sodium silicate was used assilicon oxide source. An aqueous solution of sodium silicate was treatedwith ion-exchange resin to exchange sodium ions with hydrogen ions. Theamount of silica on the surface of carbon is about 7-16% and preferablyabout 8-13% and more preferably about 10-12 wt %, of the total powder.

D. Silica coated carbon was further modified with sulfur molecules, toincrease their mercury removal capacity. Addition of sulfur was achievedby mixing elemental sulfur with the silica coated carbon, and heating itunder inert atmosphere. Other chemicals which can also be used forsulfur addition are mercaptosilane, mercapto acetic acid, and calciumpolysulfide. The amount of sulfur is about 0.4-6% and preferably about0.75-4% and more preferably about 2-3%.

E. The unique feature of the present sorbent is the presence of silicacoating on the surface of carbon. It leads to enhancement in the mercuryremoval efficiency of the substrate, from the flue gas. This also leadsto less adsorption of AEA on the surface of carbon, leading to a lowfoam index, and making it more concrete friendly. The AEA molecules aretypically an aqueous mixture of anionic surfactants. In concrete, AEAmolecules have their hydrophobic non-polar end group aligned toward theinterior of the air bubble, while the polar end group is toward eitherwater or the cement surface (which is also polar). This leads to thestabilization of air bubbles, hence preventing them from coalescing andleaving the system. However, when carbon is present in the system, thehydrophobic end of AEA is aligned toward the surface of carbon, due tothe non-polar nature of the carbon surface. This leads to the adsorptionof AEA on carbon. As a consequence, a lower amount of AEA is nowavailable for the stabilization of air bubbles, leading to a smallernumber of bubbles and a commensurate increase in the foam index, FIG. 1(a). In the case of the present sorbent, the thin silica coating on thesurface of the carbon particle leads to the formation of hydroxyl groupson the surface of the sorbents. This is shown in FIG. 1( b). The typicalpH of concrete mixture is in the basic range where O—H bond, of metaloxide layer, is broken and a proton (H⁺) is removed from the hydroxylgroup, leading to an overall negative charge. This negative chargerepels the negatively charged AEA, and reduces its adsorption on thesurface of the sorbent, leading to an overall reduction in the foamindex.

EXAMPLE 1 Synthesis and Performance of Carbon-Black Based Sorbent

1.a. Surface Modification with Sodium Aluminate

A typical process for introducing aluminum hydroxide groups on thesurface of carbon black is as follows: 60 g of carbon black wasdispersed in 5400 mL of water, using a high shear mixer. 1.2 g of sodiumaluminate was dissolved in 360 mL of water, in a separate container. Theaqueous solution of sodium aluminate was passed through an ion-exchangeresin (Dowex-HCR-W2), prior to the addition to carbon black slurry. ThepH of the solution was maintained between 9.7 and 9.8, using an aqueoussolution of sodium hydroxide and hydrochloric acid. The treated powderwas filtered, and dried in an oven.

1.b. Surface Modification with Sodium Silicate

Aluminum hydroxide activated carbon black was further coated withsilica. In a typical experiment 25 g of aluminum hydroxide activatedcarbon black was dispersed in 2250 mL of water using a high shear mixer.The temperature of the slurry was maintained between 75-80° C. In aseparate container 18.70 g of 28% sodium silicate solution was mixedwith 250 mL of water. The sodium silicate solution was treated withion-exchange resin, and finally added to the carbon black slurry, at therate of 4 mL/min. The pH of the solution was maintained around 4 usingaqueous solutions of sodium hydroxide and hydrochloric acid. FIG. 2shows a micrograph of carbon black after silica coating. FIG. 3 showsmicrograph of silica ash obtained by burning silica coated carbon blackin oxygen, which leaves silica residue. Note that the residue is in theform of silica shells (surface area: ˜600 m²/g). Thermo gravimetricanalysis of silica coated carbon black showed that the silica content inthe coated powder is 12-15%.

1.c Sulfur Modification of Silica Coated Carbon Black

To increase mercury removal efficiency of silica coated carbon, thepowder was treated with elemental sulfur. In a typical example, silicacoated carbon black powder was mixed with 5 wt % sulfur powder. 2 g ofthis powder was heated in a tube furnace at 400° C. for 6 hours innitrogen atmosphere. The final amount of sulfur after heat treatment was1.69%. This sample hereafter will be designated as C2. The specific areaof the sorbent was 239 m²/g. The specific surface area of unmodifiedcarbon black was 260 m²/g.

1.d Measurement of Mercury Removal Efficiency of C2

The sorbents were tested for total vapor-phase mercury removal in abaghouse scenario for plants burning Powder River Basin sub-bituminouscoal (PRB). The sorbent injection rate was 0.5 lb/Macf. The beginningmercury concentration in flue gas was between 11-16 μg/Nm³. A commercialsorbent, Darco Hg-LH, with surface area ˜500 m²/g was also tested forcomparison. FIG. 4 shows the specific mercury removal efficiency ofcarbon black, C2 and Darco Hg-LH, determined using per unit surface areaof sorbent. The efficiency is calculated by dividing change in Hgconcentration (Δμg/Nm³) with surface area (m²). The mercury removalefficiency of C2 is significantly higher than Darco Hg-LH, even thoughDarco Hg-LH has much higher surface area. This confirms that a higherinternal accessible surface is needed for high mercury removalefficiency of the sorbent.

1.e Measurement of Foam Index of C2.

The concrete friendliness of the sorbents was evaluated by the foamindex test. The fly ash used for foam index measurements was from thesame plant where the sorbents were evaluated. The AEA used for the testwas Darex-II, manufactured by Grace Construction. Initially, 1 wt %sorbent material was mixed with fly ash, to simulate the concentrationobserved in fly ash, when carbon based sorbents are used for Hg removal.Subsequently, 4 g of fly ash was mixed with 16 g of Portland cement(Type 1), which is used in concrete formulations. The mixture was thendispersed in 50 ml of water. 1 wt % solution of Darex-II in water wasadded to the slurry. The end point of addition of AEA was when stablefoam was observed for 45 seconds. FIG. 5 shows the foam index of C2 andcompares it with Darco Hg-LH. The foam index of C2 is lower than DarcoHg-LH, indicating that C2 is more concrete friendly than Darco Hg-LH.

EXAMPLE 2 Synthesis and Performance of Activated Carbon (AC-1) BasedSorbent

2.a Synthesis of Activated Carbon Based Sorbent

Activated carbon based sorbent was synthesized in a method similar tothe method used to synthesize carbon black sorbent, as described before.In a typical experiment 30 g of activated carbon (surface area: 550m²/g) was dispersed in 2700 mL of water using a high shear mixer.Subsequently, an aqueous solution of sodium aluminate (0.6 sodiumsilicate in 180 mL of water) was treated with ion-exchange resin, priorto its addition to activated carbon slurry. The pH of the slurry wasmaintained between 9.7-9.8, using aqueous solution of hydrochloric acidand sodium silicate. The treated powder was filtered, followed bydrying. Aluminum hydroxide functionalized activated carbon powder wassilica coated, using sodium silicate. In a typical experiment, 30 g offunctionalized powder was dispersed in 2700 mL of water. Sodium silicatesolution (20.28 g 28% sodium silicate solution in 262 mL of water), wastreated with ion-exchange resin, prior to its addition to carbon slurry.The temperature of the solution was maintained between 75° C. and 80° C.The pH of the slurry was kept at 4, using aqueous solutions of sodiumhydroxide and hydrochloric acid. The slurry was filtered and dried inoven. This powder is designated as C1. The surface area of this powderwas 508 m²/g. C1 sorbent was further treated with sulfur to increase itsmercury removal efficiency. In a typical experiment, C1 was mixed with 5wt % of elemental sulfur powder. The mixture was heat treated at 400° C.in inert atmosphere. This sorbent is designated as C3. The sulfurcontent of C3 is 2.99%, and surface area is 479 m²/g.

2.b Performance of C1 and C3

C1 sorbent was tested for mercury removal efficiency, using the methoddescribed above. FIG. 6 shows the mercury removal efficiency of AC-1, C1and C3, in conjunction with Darco Hg-LH. Once again, the mercury removalefficiency of C1 and C3 is higher than Darco Hg-LH, even though thesurface areas are comparable. This is due to their more open structureof this carbon than Darco Hg-LH. FIG. 7 shows the foam index of C1 andC3, and compares it with Darco Hg-LH. The foam index of C1 is ⅓^(rd)that of Darco Hg-LH, indicating that it is significantly more concretefriendly than Darco Hg-LH.

EXAMPLE 3

Synthesis and Performance of Activated Carbon (AC-2) Based SorbentAnother activated carbon (AC-2) with different surface area (600 m²/g)and particle morphology was modified to increase its mercury removalefficiency. The sorbent was synthesized in a manner similar to themethod described to synthesize C1. Silica coated sorbent synthesizedusing AC-2, is designated as C5. No sulfur modification was performedfor this carbon. The silica content for the sorbent was around 20 wt %.The surface area of the modified AC-2 was 371.8 m²/g.

FIG. 8 shows the mercury removal efficiency of AC-2 and C5. The sorbentinjection rate was 0.5 lb/Macf. The AC-2 increased significantly aftersurface modification.

The present invention clearly demonstrates that an open pore structure,with suitable surface modification can lead to significant improvementin the efficiency of the sorbent to remove mercury from flue gas.

As is well known, the formula parameters set forth herein are forexample only, such parameters can be scaled and adjusted in accordancewith the teaching of this invention. This invention has been describedwith respect to preferred embodiments. However, those skilled in the artwill recognize, modifications and variations in the specific detailswhich have been described and illustrated may be restored to, withoutdeparting from the sprit and scope of the invention as defined in theappended claims.

1. A sorbent to remove mercury from a gas, comprising: a) an inorganicsubstrate; b) a tying layer disposed on the inorganic substrate; and c)a metal oxide layer disposed on the tying layer; wherein the tying layeris disposed between the inorganic substrate and the metal oxide layer toincrease adhesion between the inorganic substrate and the metal oxidelayer; and wherein said tying layer comprises at least one of aluminumhydroxide, titanium hydroxide, magnesium hydroxide, iron hydroxide, andcopper hydroxide.
 2. The sorbent as claimed in claim 1 wherein saidsubstrate is composed of at least one of: carbon, ceramic oxides,ceramic non-oxides and clay-based particles.
 3. The sorbent as claimedin claim 1 wherein said substrate comprises activated carbon.
 4. Thesorbent as claimed in claim 1, wherein said metal oxide layer comprisesat least one of silicon oxide, aluminum oxide, titanium oxide, ironoxide and tin oxide.
 5. The sorbent as claimed in claim 1, wherein saidmetal oxide layer further includes sulfur molecules to increase itsmercury removal efficiency.
 6. The sorbent as claimed in claim 5,wherein said sulfur molecules comprise at least one of: elementalsulfur, mercaptosilane, and calcium polysulfide.
 7. The sorbent asclaimed in claim 1, wherein the concentration of metal oxide layer onthe substrate comprises 7-16 wt % of the total sorbent.
 8. The sorbentas claimed in claim 1, wherein the concentration of metal oxide layer onthe substrate comprises 10-12 wt % of the total sorbent.
 9. An improvedcarbon-based sorbent to remove mercury from flue gas, comprising: a) acarbon substrate b) a tying layer disposed on the carbon substrate c) ametal oxide layer disposed on the tying layer wherein the tying layer isdisposed between the carbon substrate and the metal oxide layer toincrease adhesion between the carbon substrate and the metal oxidelayer; and wherein said tying layer comprises at least one of aluminumhydroxide, titanium hydroxide, magnesium hydroxide, iron hydroxide, andcopper hydroxide.
 10. The sorbent as claimed in claim 9 wherein saidcarbon substrate comprises activated carbon.
 11. The sorbent as claimedin claim 9, wherein said metal oxide layer comprises at least one ofsilicon oxide, aluminum oxide, titanium oxide, iron oxide and tin oxide.12. The sorbent as claimed in claim 9, wherein said metal oxide layerfurther includes sulfur molecules to increase its mercury removalefficiency.
 13. The sorbent as claimed in claim 12, wherein said sulfurmolecules comprise at least one of: elemental sulfur, mercaptosilane,and calcium polysulfide.
 14. The sorbent as claimed in claim 9, whereinthe concentration of metal oxide layer on the substrate comprises 7-16%of the total sorbent.
 15. A process for producing a sorbent to removemercury from gas, comprising: a) providing an inorganic substrate; b)depositing a tying layer on the inorganic substrate to increase theadhesion between the inorganic substrate and subsequent layers, saidtying layer comprising at least one of aluminum hydroxide, titaniumhydroxide, magnesium hydroxide, iron hydroxide, and copper hydroxide;and c) depositing a metal oxide layer on the surface of the tying layer,said metal oxide layer being of a different metal from that of saidtying layer.
 16. The process as claimed in claim 15, wherein saidinorganic substrate comprises activated carbon.
 17. The process asclaimed in claim 15, wherein said metal oxide layer comprises silica.18. The process as claimed in claim 15, wherein said metal oxide layerfurther includes sulfur molecules to increase its mercury removalefficiency.
 19. The process as claimed in claim 18, wherein said sulfurmolecules comprise at least one of: elemental sulfur, mercaptosilane,and calcium polysulfide.